Learn C++ in Y Minutes

learn.cpp

// https://learnxinyminutes.com/docs/c++/
//////////////////
// Comparison to C
//////////////////

// C++ is _almost_ a superset of C and shares its basic syntax for
// variable declarations, primitive types, and functions.

// Just like in C, your program's entry point is a function called
// main with an integer return type.
// This value serves as the program's exit status.
// See https://en.wikipedia.org/wiki/Exit_status for more information.
int main(int argc, char** argv)
{
    // Command line arguments are passed in by argc and argv in the same way
    // they are in C.
    // argc indicates the number of arguments,
    // and argv is an array of C-style strings (char*)
    // representing the arguments.
    // The first argument is the name by which the program was called.
    // argc and argv can be omitted if you do not care about arguments,
    // giving the function signature of int main()

    // An exit status of 0 indicates success.
    return 0;
}

// However, C++ varies in some of the following ways:

// In C++, character literals are chars
sizeof('c') == sizeof(char) == 1

// In C, character literals are ints
sizeof('c') == sizeof(int)


// C++ has strict prototyping
void func(); // function which accepts no arguments

// In C
void func(); // function which may accept any number of arguments

// Use nullptr instead of NULL in C++
int* ip = nullptr;

// C standard headers are available in C++.
// C headers end in .h, while
// C++ headers are prefixed with "c" and have no ".h" suffix.

// The C++ standard version:
#include <cstdio>

// The C standard version:
#include <stdio.h>

int main()
{
    printf("Hello, world!\n");
    return 0;
}

///////////////////////
// Function overloading
///////////////////////

// C++ supports function overloading
// provided each function takes different parameters.

void print(char const* myString)
{
    printf("String %s\n", myString);
}

void print(int myInt)
{
    printf("My int is %d", myInt);
}

int main()
{
    print("Hello"); // Resolves to void print(const char*)
    print(15); // Resolves to void print(int)
}

/////////////////////////////
// Default function arguments
/////////////////////////////

// You can provide default arguments for a function
// if they are not provided by the caller.

void doSomethingWithInts(int a = 1, int b = 4)
{
    // Do something with the ints here
}

int main()
{
    doSomethingWithInts();      // a = 1,  b = 4
    doSomethingWithInts(20);    // a = 20, b = 4
    doSomethingWithInts(20, 5); // a = 20, b = 5
}

// Default arguments must be at the end of the arguments list.

void invalidDeclaration(int a = 1, int b) // Error!
{
}


/////////////
// Namespaces
/////////////

// Namespaces provide separate scopes for variable, function,
// and other declarations.
// Namespaces can be nested.

namespace First {
    namespace Nested {
        void foo()
        {
            printf("This is First::Nested::foo\n");
        }
    } // end namespace Nested
} // end namespace First

namespace Second {
    void foo()
    {
        printf("This is Second::foo\n");
    }
    void bar()
    {
        printf("This is Second::bar\n");
    }
}

void foo()
{
    printf("This is global foo\n");
}

int main()
{
    // Includes all symbols from namespace Second into the current scope. Note
    // that while bar() works, simply using foo() no longer works, since it is
    // now ambiguous whether we're calling the foo in namespace Second or the
    // top level.
    using namespace Second;

    bar(); // prints "This is Second::bar"
    Second::foo(); // prints "This is Second::foo"
    First::Nested::foo(); // prints "This is First::Nested::foo"
    ::foo(); // prints "This is global foo"
}

///////////////
// Input/Output
///////////////

// C++ input and output uses streams
// cin, cout, and cerr represent stdin, stdout, and stderr.
// << is the insertion operator and >> is the extraction operator.

#include <iostream> // Include for I/O streams

using namespace std; // Streams are in the std namespace (standard library)

int main()
{
   int myInt;

   // Prints to stdout (or terminal/screen)
   cout << "Enter your favorite number:\n";
   // Takes in input
   cin >> myInt;

   // cout can also be formatted
   cout << "Your favorite number is " << myInt << '\n';
   // prints "Your favorite number is <myInt>"

   cerr << "Used for error messages";
}

//////////
// Strings
//////////

// Strings in C++ are objects and have many member functions
#include <string>

using namespace std; // Strings are also in the namespace std (standard library)

string myString = "Hello";
string myOtherString = " World";

// + is used for concatenation.
cout << myString + myOtherString; // "Hello World"

cout << myString + " You"; // "Hello You"

// C++ strings are mutable.
myString.append(" Dog");
cout << myString; // "Hello Dog"


/////////////
// References
/////////////

// In addition to pointers like the ones in C,
// C++ has _references_.
// These are pointer types that cannot be reassigned once set
// and cannot be null.
// They also have the same syntax as the variable itself:
// No * is needed for dereferencing and
// & (address of) is not used for assignment.

using namespace std;

string foo = "I am foo";
string bar = "I am bar";


string& fooRef = foo; // This creates a reference to foo.
fooRef += ". Hi!"; // Modifies foo through the reference
cout << fooRef; // Prints "I am foo. Hi!"

// Doesn't reassign "fooRef". This is the same as "foo = bar", and
//   foo == "I am bar"
// after this line.
cout << &fooRef << endl; //Prints the address of foo
fooRef = bar;
cout << &fooRef << endl; //Still prints the address of foo
cout << fooRef;  // Prints "I am bar"

// The address of fooRef remains the same, i.e. it is still referring to foo.


const string& barRef = bar; // Create a const reference to bar.
// Like C, const values (and pointers and references) cannot be modified.
barRef += ". Hi!"; // Error, const references cannot be modified.

// Sidetrack: Before we talk more about references, we must introduce a concept
// called a temporary object. Suppose we have the following code:
string tempObjectFun() { ... }
string retVal = tempObjectFun();

// What happens in the second line is actually:
//   - a string object is returned from tempObjectFun
//   - a new string is constructed with the returned object as argument to the
//     constructor
//   - the returned object is destroyed
// The returned object is called a temporary object. Temporary objects are
// created whenever a function returns an object, and they are destroyed at the
// end of the evaluation of the enclosing expression (Well, this is what the
// standard says, but compilers are allowed to change this behavior. Look up
// "return value optimization" if you're into this kind of details). So in this
// code:
foo(bar(tempObjectFun()))

// assuming foo and bar exist, the object returned from tempObjectFun is
// passed to bar, and it is destroyed before foo is called.

// Now back to references. The exception to the "at the end of the enclosing
// expression" rule is if a temporary object is bound to a const reference, in
// which case its life gets extended to the current scope:

void constReferenceTempObjectFun() {
  // constRef gets the temporary object, and it is valid until the end of this
  // function.
  const string& constRef = tempObjectFun();
  ...
}

// Another kind of reference introduced in C++11 is specifically for temporary
// objects. You cannot have a variable of its type, but it takes precedence in
// overload resolution:

void someFun(string& s) { ... }  // Regular reference
void someFun(string&& s) { ... }  // Reference to temporary object

string foo;
someFun(foo);  // Calls the version with regular reference
someFun(tempObjectFun());  // Calls the version with temporary reference

// For example, you will see these two versions of constructors for
// std::basic_string:
basic_string(const basic_string& other);
basic_string(basic_string&& other);

// Idea being if we are constructing a new string from a temporary object (which
// is going to be destroyed soon anyway), we can have a more efficient
// constructor that "salvages" parts of that temporary string. You will see this
// concept referred to as "move semantics".

/////////////////////
// Enums
/////////////////////

// Enums are a way to assign a value to a constant most commonly used for
// easier visualization and reading of code
enum ECarTypes
{
  Sedan,
  Hatchback,
  SUV,
  Wagon
};

ECarTypes GetPreferredCarType()
{
    return ECarTypes::Hatchback;
}

// As of C++11 there is an easy way to assign a type to the enum which can be
// useful in serialization of data and converting enums back-and-forth between
// the desired type and their respective constants
enum ECarTypes : uint8_t
{
  Sedan, // 0
  Hatchback, // 1
  SUV = 254, // 254
  Hybrid // 255
};

void WriteByteToFile(uint8_t InputValue)
{
    // Serialize the InputValue to a file
}

void WritePreferredCarTypeToFile(ECarTypes InputCarType)
{
    // The enum is implicitly converted to a uint8_t due to its declared enum type
    WriteByteToFile(InputCarType);
}

// On the other hand you may not want enums to be accidentally cast to an integer
// type or to other enums so it is instead possible to create an enum class which
// won't be implicitly converted
enum class ECarTypes : uint8_t
{
  Sedan, // 0
  Hatchback, // 1
  SUV = 254, // 254
  Hybrid // 255
};

void WriteByteToFile(uint8_t InputValue)
{
    // Serialize the InputValue to a file
}

void WritePreferredCarTypeToFile(ECarTypes InputCarType)
{
    // Won't compile even though ECarTypes is a uint8_t due to the enum
    // being declared as an "enum class"!
    WriteByteToFile(InputCarType);
}

//////////////////////////////////////////
// Classes and object-oriented programming
//////////////////////////////////////////

// First example of classes
#include <iostream>

// Declare a class.
// Classes are usually declared in header (.h or .hpp) files.
class Dog {
    // Member variables and functions are private by default.
    std::string name;
    int weight;

// All members following this are public
// until "private:" or "protected:" is found.
public:

    // Default constructor
    Dog();

    // Member function declarations (implementations to follow)
    // Note that we use std::string here instead of placing
    // using namespace std;
    // above.
    // Never put a "using namespace" statement in a header.
    void setName(const std::string& dogsName);

    void setWeight(int dogsWeight);

    // Functions that do not modify the state of the object
    // should be marked as const.
    // This allows you to call them if given a const reference to the object.
    // Also note the functions must be explicitly declared as _virtual_
    // in order to be overridden in derived classes.
    // Functions are not virtual by default for performance reasons.
    virtual void print() const;

    // Functions can also be defined inside the class body.
    // Functions defined as such are automatically inlined.
    void bark() const { std::cout << name << " barks!\n"; }

    // Along with constructors, C++ provides destructors.
    // These are called when an object is deleted or falls out of scope.
    // This enables powerful paradigms such as RAII
    // (see below)
    // The destructor should be virtual if a class is to be derived from;
    // if it is not virtual, then the derived class' destructor will
    // not be called if the object is destroyed through a base-class reference
    // or pointer.
    virtual ~Dog();

}; // A semicolon must follow the class definition.

// Class member functions are usually implemented in .cpp files.
Dog::Dog()
{
    std::cout << "A dog has been constructed\n";
}

// Objects (such as strings) should be passed by reference
// if you are modifying them or const reference if you are not.
void Dog::setName(const std::string& dogsName)
{
    name = dogsName;
}

void Dog::setWeight(int dogsWeight)
{
    weight = dogsWeight;
}

// Notice that "virtual" is only needed in the declaration, not the definition.
void Dog::print() const
{
    std::cout << "Dog is " << name << " and weighs " << weight << "kg\n";
}

Dog::~Dog()
{
    std::cout << "Goodbye " << name << '\n';
}

int main() {
    Dog myDog; // prints "A dog has been constructed"
    myDog.setName("Barkley");
    myDog.setWeight(10);
    myDog.print(); // prints "Dog is Barkley and weighs 10 kg"
    return 0;
} // prints "Goodbye Barkley"

// Inheritance:

// This class inherits everything public and protected from the Dog class
// as well as private but may not directly access private members/methods
// without a public or protected method for doing so
class OwnedDog : public Dog {

public:
    void setOwner(const std::string& dogsOwner);

    // Override the behavior of the print function for all OwnedDogs. See
    // https://en.wikipedia.org/wiki/Polymorphism_(computer_science)#Subtyping
    // for a more general introduction if you are unfamiliar with
    // subtype polymorphism.
    // The override keyword is optional but makes sure you are actually
    // overriding the method in a base class.
    void print() const override;

private:
    std::string owner;
};

// Meanwhile, in the corresponding .cpp file:

void OwnedDog::setOwner(const std::string& dogsOwner)
{
    owner = dogsOwner;
}

void OwnedDog::print() const
{
    Dog::print(); // Call the print function in the base Dog class
    std::cout << "Dog is owned by " << owner << '\n';
    // Prints "Dog is <name> and weights <weight>"
    //        "Dog is owned by <owner>"
}

//////////////////////////////////////////
// Initialization and Operator Overloading
//////////////////////////////////////////

// In C++ you can overload the behavior of operators such as +, -, *, /, etc.
// This is done by defining a function which is called
// whenever the operator is used.

#include <iostream>
using namespace std;

class Point {
public:
    // Member variables can be given default values in this manner.
    double x = 0;
    double y = 0;

    // Define a default constructor which does nothing
    // but initialize the Point to the default value (0, 0)
    Point() { };

    // The following syntax is known as an initialization list
    // and is the proper way to initialize class member values
    Point (double a, double b) :
        x(a),
        y(b)
    { /* Do nothing except initialize the values */ }

    // Overload the + operator.
    Point operator+(const Point& rhs) const;

    // Overload the += operator
    Point& operator+=(const Point& rhs);

    // It would also make sense to add the - and -= operators,
    // but we will skip those for brevity.
};

Point Point::operator+(const Point& rhs) const
{
    // Create a new point that is the sum of this one and rhs.
    return Point(x + rhs.x, y + rhs.y);
}

// It's good practice to return a reference to the leftmost variable of
// an assignment. `(a += b) == c` will work this way.
Point& Point::operator+=(const Point& rhs)
{
    x += rhs.x;
    y += rhs.y;

    // `this` is a pointer to the object, on which a method is called.
    return *this;
}

int main () {
    Point up (0,1);
    Point right (1,0);
    // This calls the Point + operator
    // Point up calls the + (function) with right as its parameter
    Point result = up + right;
    // Prints "Result is upright (1,1)"
    cout << "Result is upright (" << result.x << ',' << result.y << ")\n";
    return 0;
}

/////////////////////
// Templates
/////////////////////

// Templates in C++ are mostly used for generic programming, though they are
// much more powerful than generic constructs in other languages. They also
// support explicit and partial specialization and functional-style type
// classes; in fact, they are a Turing-complete functional language embedded
// in C++!

// We start with the kind of generic programming you might be familiar with. To
// define a class or function that takes a type parameter:
template<class T>
class Box {
public:
    // In this class, T can be used as any other type.
    void insert(const T&) { ... }
};

// During compilation, the compiler actually generates copies of each template
// with parameters substituted, so the full definition of the class must be
// present at each invocation. This is why you will see template classes defined
// entirely in header files.

// To instantiate a template class on the stack:
Box<int> intBox;

// and you can use it as you would expect:
intBox.insert(123);

// You can, of course, nest templates:
Box<Box<int> > boxOfBox;
boxOfBox.insert(intBox);

// Until C++11, you had to place a space between the two '>'s, otherwise '>>'
// would be parsed as the right shift operator.

// You will sometimes see
//   template<typename T>
// instead. The 'class' keyword and 'typename' keywords are _mostly_
// interchangeable in this case. For the full explanation, see
//   https://en.wikipedia.org/wiki/Typename
// (yes, that keyword has its own Wikipedia page).

// Similarly, a template function:
template<class T>
void barkThreeTimes(const T& input)
{
    input.bark();
    input.bark();
    input.bark();
}

// Notice that nothing is specified about the type parameters here. The compiler
// will generate and then type-check every invocation of the template, so the
// above function works with any type 'T' that has a const 'bark' method!

Dog fluffy;
fluffy.setName("Fluffy")
barkThreeTimes(fluffy); // Prints "Fluffy barks" three times.

// Template parameters don't have to be classes:
template<int Y>
void printMessage() {
  cout << "Learn C++ in " << Y << " minutes!" << endl;
}

// And you can explicitly specialize templates for more efficient code. Of
// course, most real-world uses of specialization are not as trivial as this.
// Note that you still need to declare the function (or class) as a template
// even if you explicitly specified all parameters.
template<>
void printMessage<10>() {
  cout << "Learn C++ faster in only 10 minutes!" << endl;
}

printMessage<20>();  // Prints "Learn C++ in 20 minutes!"
printMessage<10>();  // Prints "Learn C++ faster in only 10 minutes!"


/////////////////////
// Exception Handling
/////////////////////

// The standard library provides a few exception types
// (see https://en.cppreference.com/w/cpp/error/exception)
// but any type can be thrown as an exception
#include <exception>
#include <stdexcept>

// All exceptions thrown inside the _try_ block can be caught by subsequent
// _catch_ handlers.
try {
    // Do not allocate exceptions on the heap using _new_.
    throw std::runtime_error("A problem occurred");
}

// Catch exceptions by const reference if they are objects
catch (const std::exception& ex)
{
    std::cout << ex.what();
}

// Catches any exception not caught by previous _catch_ blocks
catch (...)
{
    std::cout << "Unknown exception caught";
    throw; // Re-throws the exception
}

///////
// RAII
///////

// RAII stands for "Resource Acquisition Is Initialization".
// It is often considered the most powerful paradigm in C++
// and is the simple concept that a constructor for an object
// acquires that object's resources and the destructor releases them.

// To understand how this is useful,
// consider a function that uses a C file handle:
void doSomethingWithAFile(const char* filename)
{
    // To begin with, assume nothing can fail.

    FILE* fh = fopen(filename, "r"); // Open the file in read mode.

    doSomethingWithTheFile(fh);
    doSomethingElseWithIt(fh);

    fclose(fh); // Close the file handle.
}

// Unfortunately, things are quickly complicated by error handling.
// Suppose fopen can fail, and that doSomethingWithTheFile and
// doSomethingElseWithIt return error codes if they fail.
//  (Exceptions are the preferred way of handling failure,
//   but some programmers, especially those with a C background,
//   disagree on the utility of exceptions).
// We now have to check each call for failure and close the file handle
// if a problem occurred.
bool doSomethingWithAFile(const char* filename)
{
    FILE* fh = fopen(filename, "r"); // Open the file in read mode
    if (fh == nullptr) // The returned pointer is null on failure.
        return false; // Report that failure to the caller.

    // Assume each function returns false if it failed
    if (!doSomethingWithTheFile(fh)) {
        fclose(fh); // Close the file handle so it doesn't leak.
        return false; // Propagate the error.
    }
    if (!doSomethingElseWithIt(fh)) {
        fclose(fh); // Close the file handle so it doesn't leak.
        return false; // Propagate the error.
    }

    fclose(fh); // Close the file handle so it doesn't leak.
    return true; // Indicate success
}

// C programmers often clean this up a little bit using goto:
bool doSomethingWithAFile(const char* filename)
{
    FILE* fh = fopen(filename, "r");
    if (fh == nullptr)
        return false;

    if (!doSomethingWithTheFile(fh))
        goto failure;

    if (!doSomethingElseWithIt(fh))
        goto failure;

    fclose(fh); // Close the file
    return true; // Indicate success

failure:
    fclose(fh);
    return false; // Propagate the error
}

// If the functions indicate errors using exceptions,
// things are a little cleaner, but still sub-optimal.
void doSomethingWithAFile(const char* filename)
{
    FILE* fh = fopen(filename, "r"); // Open the file in shared_ptrread mode
    if (fh == nullptr)
        throw std::runtime_error("Could not open the file.");

    try {
        doSomethingWithTheFile(fh);
        doSomethingElseWithIt(fh);
    }
    catch (...) {
        fclose(fh); // Be sure to close the file if an error occurs.
        throw; // Then re-throw the exception.
    }

    fclose(fh); // Close the file
    // Everything succeeded
}

// Compare this to the use of C++'s file stream class (fstream)
// fstream uses its destructor to close the file.
// Recall from above that destructors are automatically called
// whenever an object falls out of scope.
void doSomethingWithAFile(const std::string& filename)
{
    // ifstream is short for input file stream
    std::ifstream fh(filename); // Open the file

    // Do things with the file
    doSomethingWithTheFile(fh);
    doSomethingElseWithIt(fh);

} // The file is automatically closed here by the destructor

// This has _massive_ advantages:
// 1. No matter what happens,
//    the resource (in this case the file handle) will be cleaned up.
//    Once you write the destructor correctly,
//    It is _impossible_ to forget to close the handle and leak the resource.
// 2. Note that the code is much cleaner.
//    The destructor handles closing the file behind the scenes
//    without you having to worry about it.
// 3. The code is exception safe.
//    An exception can be thrown anywhere in the function and cleanup
//    will still occur.

// All idiomatic C++ code uses RAII extensively for all resources.
// Additional examples include
// - Memory using unique_ptr and shared_ptr
// - Containers - the standard library linked list,
//   vector (i.e. self-resizing array), hash maps, and so on
//   all automatically destroy their contents when they fall out of scope.
// - Mutexes using lock_guard and unique_lock


/////////////////////
// Smart Pointer
/////////////////////

// Generally a smart pointer is a class which wraps a "raw pointer" (usage of "new"
// respectively malloc/calloc in C). The goal is to be able to
// manage the lifetime of the object being pointed to without ever needing to explicitly delete 
// the object. The term itself simply describes a set of pointers with the
// mentioned abstraction.
// Smart pointers should preferred over raw pointers, to prevent
// risky memory leaks, which happen if you forget to delete an object.

// Usage of a raw pointer:
Dog* ptr = new Dog();
ptr->bark();
delete ptr;

// By using a smart pointer, you don't have to worry about the deletion
// of the object anymore.
// A smart pointer describes a policy, to count the references to the
// pointer. The object gets destroyed when the last
// reference to the object gets destroyed.

// Usage of "std::shared_ptr":
void foo()
{
// It's no longer necessary to delete the Dog.
std::shared_ptr<Dog> doggo(new Dog());
doggo->bark();
}

// Beware of possible circular references!!!
// There will be always a reference, so it will be never destroyed!
std::shared_ptr<Dog> doggo_one(new Dog());
std::shared_ptr<Dog> doggo_two(new Dog());
doggo_one = doggo_two; // p1 references p2
doggo_two = doggo_one; // p2 references p1

// There are several kinds of smart pointers. 
// The way you have to use them is always the same.
// This leads us to the question: when should we use each kind of smart pointer?
// std::unique_ptr - use it when you just want to hold one reference to
// the object.
// std::shared_ptr - use it when you want to hold multiple references to the
// same object and want to make sure that it's deallocated
// when all references are gone.
// std::weak_ptr - use it when you want to access
// the underlying object of a std::shared_ptr without causing that object to stay allocated.
// Weak pointers are used to prevent circular referencing.


/////////////////////
// Containers
/////////////////////

// Containers or the Standard Template Library are some predefined templates.
// They manage the storage space for its elements and provide
// member functions to access and manipulate them.

// Few containers are as follows:

// Vector (Dynamic array)
// Allow us to Define the Array or list of objects at run time
#include <vector>
string val;
vector<string> my_vector; // initialize the vector
cin >> val;
my_vector.push_back(val); // will push the value of 'val' into vector ("array") my_vector
my_vector.push_back(val); // will push the value into the vector again (now having two elements)

// To iterate through a vector we have 2 choices:
// Either classic looping (iterating through the vector from index 0 to its last index):
for (int i = 0; i < my_vector.size(); i++) {
    cout << my_vector[i] << endl; // for accessing a vector's element we can use the operator []
}

// or using an iterator:
vector<string>::iterator it; // initialize the iterator for vector
for (it = my_vector.begin(); it != my_vector.end(); ++it) {
    cout << *it  << endl;
}

// Set
// Sets are containers that store unique elements following a specific order.
// Set is a very useful container to store unique values in sorted order
// without any other functions or code.

#include<set>
set<int> ST;    // Will initialize the set of int data type
ST.insert(30);  // Will insert the value 30 in set ST
ST.insert(10);  // Will insert the value 10 in set ST
ST.insert(20);  // Will insert the value 20 in set ST
ST.insert(30);  // Will insert the value 30 in set ST
// Now elements of sets are as follows
//  10 20 30

// To erase an element
ST.erase(20);  // Will erase element with value 20
// Set ST: 10 30
// To iterate through Set we use iterators
set<int>::iterator it;
for(it=ST.begin();it!=ST.end();it++) {
    cout << *it << endl;
}
// Output:
// 10
// 30

// To clear the complete container we use Container_name.clear()
ST.clear();
cout << ST.size();  // will print the size of set ST
// Output: 0

// NOTE: for duplicate elements we can use multiset
// NOTE: For hash sets, use unordered_set. They are more efficient but
// do not preserve order. unordered_set is available since C++11

// Map
// Maps store elements formed by a combination of a key value
// and a mapped value, following a specific order.

#include<map>
map<char, int> mymap;  // Will initialize the map with key as char and value as int

mymap.insert(pair<char,int>('A',1));
// Will insert value 1 for key A
mymap.insert(pair<char,int>('Z',26));
// Will insert value 26 for key Z

// To iterate
map<char,int>::iterator it;
for (it=mymap.begin(); it!=mymap.end(); ++it)
    std::cout << it->first << "->" << it->second << std::endl;
// Output:
// A->1
// Z->26

// To find the value corresponding to a key
it = mymap.find('Z');
cout << it->second;

// Output: 26

// NOTE: For hash maps, use unordered_map. They are more efficient but do
// not preserve order. unordered_map is available since C++11.

// Containers with object keys of non-primitive values (custom classes) require
// compare function in the object itself or as a function pointer. Primitives
// have default comparators, but you can override it.
class Foo {
public:
    int j;
    Foo(int a) : j(a) {}
};
struct compareFunction {
    bool operator()(const Foo& a, const Foo& b) const {
        return a.j < b.j;
    }
};
// this isn't allowed (although it can vary depending on compiler)
// std::map<Foo, int> fooMap;
std::map<Foo, int, compareFunction> fooMap;
fooMap[Foo(1)]  = 1;
fooMap.find(Foo(1)); //true


///////////////////////////////////////
// Lambda Expressions (C++11 and above)
///////////////////////////////////////

// lambdas are a convenient way of defining an anonymous function
// object right at the location where it is invoked or passed as
// an argument to a function.

// For example, consider sorting a vector of pairs using the second
// value of the pair

vector<pair<int, int> > tester;
tester.push_back(make_pair(3, 6));
tester.push_back(make_pair(1, 9));
tester.push_back(make_pair(5, 0));

// Pass a lambda expression as third argument to the sort function
// sort is from the <algorithm> header

sort(tester.begin(), tester.end(), [](const pair<int, int>& lhs, const pair<int, int>& rhs) {
        return lhs.second < rhs.second;
    });

// Notice the syntax of the lambda expression,
// [] in the lambda is used to "capture" variables
// The "Capture List" defines what from the outside of the lambda should be available inside the function body and how.
// It can be either:
//     1. a value : [x]
//     2. a reference : [&x]
//     3. any variable currently in scope by reference [&]
//     4. same as 3, but by value [=]
// Example:

vector<int> dog_ids;
// number_of_dogs = 3;
for(int i = 0; i < 3; i++) {
    dog_ids.push_back(i);
}

int weight[3] = {30, 50, 10};

// Say you want to sort dog_ids according to the dogs' weights
// So dog_ids should in the end become: [2, 0, 1]

// Here's where lambda expressions come in handy

sort(dog_ids.begin(), dog_ids.end(), [&weight](const int &lhs, const int &rhs) {
        return weight[lhs] < weight[rhs];
    });
// Note we captured "weight" by reference in the above example.
// More on Lambdas in C++ : https://stackoverflow.com/questions/7627098/what-is-a-lambda-expression-in-c11

///////////////////////////////
// Range For (C++11 and above)
///////////////////////////////

// You can use a range for loop to iterate over a container
int arr[] = {1, 10, 3};

for(int elem: arr){
    cout << elem << endl;
}

// You can use "auto" and not worry about the type of the elements of the container
// For example:

for(auto elem: arr) {
    // Do something with each element of arr
}

/////////////////////
// Fun stuff
/////////////////////

// Aspects of C++ that may be surprising to newcomers (and even some veterans).
// This section is, unfortunately, wildly incomplete; C++ is one of the easiest
// languages with which to shoot yourself in the foot.

// You can override private methods!
class Foo {
  virtual void bar();
};
class FooSub : public Foo {
  virtual void bar();  // Overrides Foo::bar!
};


// 0 == false == NULL (most of the time)!
bool* pt = new bool;
*pt = 0; // Sets the value points by 'pt' to false.
pt = 0;  // Sets 'pt' to the null pointer. Both lines compile without warnings.

// nullptr is supposed to fix some of that issue:
int* pt2 = new int;
*pt2 = nullptr; // Doesn't compile
pt2 = nullptr;  // Sets pt2 to null.

// There is an exception made for bools.
// This is to allow you to test for null pointers with if(!ptr),
// but as a consequence you can assign nullptr to a bool directly!
*pt = nullptr;  // This still compiles, even though '*pt' is a bool!


// '=' != '=' != '='!
// Calls Foo::Foo(const Foo&) or some variant (see move semantics) copy
// constructor.
Foo f2;
Foo f1 = f2;

// Calls Foo::Foo(const Foo&) or variant, but only copies the 'Foo' part of
// 'fooSub'. Any extra members of 'fooSub' are discarded. This sometimes
// horrifying behavior is called "object slicing."
FooSub fooSub;
Foo f1 = fooSub;

// Calls Foo::operator=(Foo&) or variant.
Foo f1;
f1 = f2;


///////////////////////////////////////
// Tuples (C++11 and above)
///////////////////////////////////////

#include<tuple>

// Conceptually, Tuples are similar to old data structures (C-like structs)
// but instead of having named data members,
// its elements are accessed by their order in the tuple.

// We start with constructing a tuple.
// Packing values into tuple
auto first = make_tuple(10, 'A');
const int maxN = 1e9;
const int maxL = 15;
auto second = make_tuple(maxN, maxL);

// Printing elements of 'first' tuple
cout << get<0>(first) << " " << get<1>(first) << '\n'; //prints : 10 A

// Printing elements of 'second' tuple
cout << get<0>(second) << " " << get<1>(second) << '\n'; // prints: 1000000000 15

// Unpacking tuple into variables

int first_int;
char first_char;
tie(first_int, first_char) = first;
cout << first_int << " " << first_char << '\n';  // prints : 10 A

// We can also create tuple like this.

tuple<int, char, double> third(11, 'A', 3.14141);
// tuple_size returns number of elements in a tuple (as a constexpr)

cout << tuple_size<decltype(third)>::value << '\n'; // prints: 3

// tuple_cat concatenates the elements of all the tuples in the same order.

auto concatenated_tuple = tuple_cat(first, second, third);
// concatenated_tuple becomes = (10, 'A', 1e9, 15, 11, 'A', 3.14141)

cout << get<0>(concatenated_tuple) << '\n'; // prints: 10
cout << get<3>(concatenated_tuple) << '\n'; // prints: 15
cout << get<5>(concatenated_tuple) << '\n'; // prints: 'A'


///////////////////////////////////
// Logical and Bitwise operators
//////////////////////////////////

// Most of the operators in C++ are same as in other languages

// Logical operators

// C++ uses Short-circuit evaluation for boolean expressions, i.e, the second argument is executed or
// evaluated only if the first argument does not suffice to determine the value of the expression

true && false // Performs **logical and** to yield false
true || false // Performs **logical or** to yield true
! true        // Performs **logical not** to yield false

// Instead of using symbols equivalent keywords can be used
true and false // Performs **logical and** to yield false
true or false  // Performs **logical or** to yield true
not true       // Performs **logical not** to yield false

// Bitwise operators

// **<<** Left Shift Operator
// << shifts bits to the left
4 << 1 // Shifts bits of 4 to left by 1 to give 8
// x << n can be thought as x * 2^n


// **>>** Right Shift Operator
// >> shifts bits to the right
4 >> 1 // Shifts bits of 4 to right by 1 to give 2
// x >> n can be thought as x / 2^n

~4    // Performs a bitwise not
4 | 3 // Performs bitwise or
4 & 3 // Performs bitwise and
4 ^ 3 // Performs bitwise xor

// Equivalent keywords are
compl 4    // Performs a bitwise not
4 bitor 3  // Performs bitwise or
4 bitand 3 // Performs bitwise and
4 xor 3    // Performs bitwise xor